Unify media_key and item_key — use item_key everywhere

media_key was a lossy shortening of item_key (libraryId:lastSegment) that
introduced a real collision bug: two TV episodes from different series with
the same filename would share the same media_key and each other's tags.

- DB migration converts existing media_tags rows from short format to full
  item_key by joining against media_items; ambiguous/orphaned rows are dropped
- media_tags column renamed media_key → item_key
- Removed itemKeyToMediaKey() from scanner; reconcileAndPrune now passes
  item_key directly to reKeyMediaItem
- DB reader functions (tv, movies, games) now expose item_key on returned
  entities; frontend components use entity.item_key instead of constructing
  the short libraryId:id form
- MixedView now constructs the full mixed_file: item_key format
- Tag API renamed mediaKey param → itemKey throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/games/GamesView.tsx

@@ -81,7 +81,7 @@ export default function GamesView({ libraryId }: Props) {
       if (!searchMatch) return false
       if (selectedTagIds.size > 0) {
         return item.games.some((g) => {
-          const gameTags = assignments[`${libraryId}:${g.id}`] ?? []
+          const gameTags = assignments[g.item_key!] ?? []
           return [...selectedTagIds].every((id) => gameTags.includes(id))
         })
       }
@@ -89,7 +89,7 @@ export default function GamesView({ libraryId }: Props) {
     }
     if (search && !item.title.toLowerCase().includes(search.toLowerCase())) return false
     if (selectedTagIds.size > 0) {
-      const gameTags = assignments[`${libraryId}:${item.id}`] ?? []
+      const gameTags = assignments[item.item_key!] ?? []
       if (![...selectedTagIds].every((id) => gameTags.includes(id))) return false
     }
     return true